We had female littermates that were with us soon after they were born in 2005. We had to euthanize the first one in 2020. In April we had to do the same with the second one. It’s awful and leaves a huge void and feeling of emptiness. We had a mobile vet come to our house on both occasions when we had to say goodbye (highly recommended this because it lessens the stress on the pet since they are not being crated up and stressed out). Despite allowing the remaining cat to smell her deceased sister in that hope that it gave her some sense of why she was going to soon disappear, I’m not sure that the remaining one ever truly had closure. Be prepared for the possibility that your feline will experience some sadness and search for the lost one. Feline grief is real. As others have said, take comfort in the fact they you gave them a good life and love up your remaining one a little extra. I have had cats and dogs my whole life and they are truly part of the family.
Sorry for your loss